Amazon is being accused of "trading on people's forgetfulness" by billing people for an annual membership for Amazon Prime after a free 30-day trial.

Online shoppers are being warned to watch out for Amazon's premium service which users say stung them with an annual membership fee without adequate warning.

Two Wellington users were shocked to discover debits of around $150 on their accounts last month, only to learn they'd chewed through a 30-day free trial of a service called Amazon Prime, then were billed for a year.

The second customer, who did not want to be named, didn't even realise the free trial had been triggered when she purchased a book a month earlier, and happened to spot the charge on her bank statement.

"It said something really strange and I couldn't make out what it was," said the woman.

"When I talked to a guy [at her New Zealand bank], before he even had a look he said, 'I'll tell you what I think it is, Amazon Prime'. He said 'this happened to me as well,' then he checked and confirmed it was Amazon.

​"My memory is I unticked [the box] but maybe I didn't properly," she said.

Others across the world claim they've been stung by the fee after their Amazon Prime trial ran out.

No one from Amazon responded to requests for comment, and the service continues despite complaints dating back to 2009.

Last year, Britain's Advertising Standards Authority banned Amazon from sending an advertisement to customers promoting a "free trial" of its Prime service.

Despite small print, customers complained they were unaware they would be charged £79 for the service after the free trial period ended and the advertisement was ruled misleading after six complaints were upheld.

The online retailer's Prime service boasts free two-day shipping, music and video streaming services, an online photo-storage service and other perks outside of the website's standard free membership.

The service costs US$99/year, so what Kiwis are charged differs based on the exchange rate.

However many of the perks, including the two-day free shipping and music streaming service, are not available to New Zealanders.

"Be careful about the Prime business – New Zealanders don't get all the perks from it, and it's a hefty one-off cost," Fitzsimons said.

The Wellington woman felt Amazon should notify people when the trial runs out and advised people against signing on for Amazon Prime.

"It's a rip-off. A whole year comes out of your account and there's no good reason for that."

Amazon offers users the option of opting-in to a notification email three days before the trial expires, but this must be done in your account settings.

"Given so many of Amazon's clients will be from other countries than the States, they need to have something clearly on there to say you don't get this to NZ," she said.

Both users were refunded, but still lost about $9, likely due to changes in the currency conversion or fees.

The Office of the Banking Ombudsmen had dealt with one case related to Amazon Prime in the 2014/15 financial year, communications advisor Emma Reilly said.

The ombudsmen deals with unresolved disputes between individuals and their banks.